An inhibitor of macrophage chemotaxis produced by neoplasms.

The accumulation of macrophages at neoplastic sites may be an important event in immunologically mediated tumor killing. The implantation of syngeneic neoplasms in mice, however, was found to depress the animal's ability to localize macrophages at inflammatory sites. A low-molecular-weight (6,000 to 10,000) factor released by growing neoplasms that inhibits the accumulation of macrophages in vivo and chemotactic responsiveness in vitro was identified. The factor is active in the inhibition of macrophages and is ineffectual at retarding the migration of polymorphonuclear leukocytes. Neoplastic cells may thus abrogate immunosurveillance by releasing products that prevent potentially tumoricidal macrophages from accumulating at sites of developing malignancies.